The correct option is
C Itself
Pronouns are the words that replace nouns. In order to fill in the blank, we must understand what reflexive pronouns are. Reflexive pronouns are pronouns ending in self/selves. These are used when the subject and the object of a sentence are the same. For example, 'I did this for myself.' Here, 'I' and 'myself' are the same, subject and object, 'myself' is a reflexive pronoun.NOTE: The sentence describes an action with the same subject and object. Hence, we need a reflexive pronoun here. 'Train' is a noun and it is singular, hence we need a third-person singular reflexive pronoun to fill in the blank.
Option A - 'Himself' can't be used for a train, as it is used for masculine gender. Hence, option A is incorrect.
Option C - 'Themselves' is a third person plural reflexive pronoun and not a third-person singular reflexive pronoun. Also, 'themselves' is used for people and not 'train'. Hence, option C is incorrect.
Option B - 'Itself' is a third-person singular reflexive pronoun, which can be used as a pronoun for 'train'. Hence, option B is the correct answer.
Option D - As option B is the correct answer, option D is incorrect.
'The train came to a halt by itself.'